This invention relates to a control rod for a nuclear reactor and particularly to attachment arrangements for securing plates of neutron absorbing material to the frame of the control rod and joints between such plates and other absorber members of the control rod.
In well-known commercial nuclear power reactors, for example as used in the Dresden Nuclear Power Station near Chicago, Ill., the nuclear fuel typically is in the form of sintered pellets contained in elongated cladding tubes. Such fuel elements are arranged in groups and supported between upper and lower tie plates in separately replaceable fuel assemblies or bundles as shown, for example, in U.S. Pat. No. 3,689,358. A sufficient number of such fuel assemblies are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core capable of self-sustained fission reaction. The core is submerged in a fluid, such as light water, which serves both as a working fluid and a neutron moderator.
Nuclear reactors are typically refueled periodically with an excess of reactivity sufficient to maintain operation throughout an operating cycle often in the order of one year in length of time. The reactor is then shut down and a fraction of the fuel assemblies, typically about one-quarter of the fuel assemblies, are replaced. The excess reactivity at the beginning of a cycle of operation requires a control system of sufficient strength to maintain the effective multiplication factor at unity during reactor operation. The control system customarily comprises neutron absorbing materials that serve to control the neutron population by nonfission absorption or capture of neutrons.
Typically, the control system includes mechanical control in the form of a plurality of control rods, containing neutron absorbing material, which are selectively insertable in the spaces or gaps among the fuel assemblies to control the reactivity, and hence the power level of operation, of the core. In a known arrangement, such as shown for example in U.S. Pat. No. 3,020,888, the control rod blades have a cross or cruciform transverse cross section whereby the blades of each control rod are insertable in the spaces between adjacent four fuel assemblies.
In a typical boiling water reactor (BWR) the control rods are of the bottom entry type (i.e. insertable from the bottom of the pressure vessel into the nuclear core).
Examples of such control rods are shown by Steven R. Specker et al in FIGS. 9A-10 of U.S. Pat. No. 4,285,769. Briefly, a control rod of this type has a structural frame including an elongated central spine with laterally extending cruciform (or four-armed) support members at each end. A plurality of sealed tubes containing neutron absorbing material (such as a powered boron compound) are positioned between the upper and lower arms of the support member and are covered with a U-shaped sheath attached to the upper and lower arms and to the central spine to form four laterally extending control rod blades.
In a version of a control rod shown in FIG. 9D of the above-mentioned U.S. Pat. No. 4,285,769, the boron material in the upper portions of the blades is replaced by a neutron absorbing material such as hafnium to provide a longer useful control rod lifetime as explained in that patent.
As illustrated in that patent, the hafnium material is shown as replacing the boron material in the absorber material containing tubes. A more practical arrangement, especially from the manufacturing point of view, is simply to place slabs or plates of hafnium metal in the upper portion of each control rod blade, the tubes of boron material beneath such absorber plates being shortened accordingly.
This arrangement provides several advantages. The plate form provides a significantly greater amount of hafnium per unit volume of control blade space as compared to hafnium in adjacent tubes of circular cross section. There are no tubes containing both hafnium and boron material to keep track of during manufacture and assembly.
However, this arrangement is not without its problems. Plates of absorber material such as hafnium metal are relatively heavy. Thus if the absorber plate is simply allowed to rest on the upper ends of the boron-containing tubes therebeneath, there is the risk that these long, slender thin-walled absorber tubes may be over-stressed and tend to bend or even buckle. This is especially true during scram of the reactor when the rapid upward acceleration of the control rod multiplies the force of the absorber plate on the tubes.
It is an object of this invention to provide an attachment between the lower edges of the upper support arms of the control rod frame and the upper edges of the absorber plates whereby the frame supports the plates.
For such attachment it is desirable to avoid welding (because of incompatible materials) and the use of rivets, screws or the like (because of risk of coming loose and/or stress/corrosion failure).
Attachment of the absorber plates to the upper support arms creates another potential problem. Usual manufacturing tolerances are such that, with the absorber plates attached to the upper support arms and the absorber tubes resting on the lower support arms, gaps may be left between the lower edges of the absorber plates and the upper end plugs of the absorber tubes. Such gaps undesirably act as "neutron windows" allowing neutrons to pass through the control blades without encountering absorber material.
It is another object of this invention to provide arrangements for avoiding such gaps or reducing such gaps to acceptable cross section area.